The best scenes in “Lee Daniels’ The Butler” — a family farewell at a bus station; a few drinks and a few dangerous glances among friends in an ordinary Washington, D.C., living room — steer clear of the White House and keep a comfortable and freeing distance from the flotilla of celebrity impersonations sailing by.

The supporting cast of “The Butler” is being described by the Weinstein Company promotional materials as “incredible,” and that’s accurate, in the primary-definition sense of the word. The casting, by and large, is not credible. Robin Williams may have it in him to play Ike Eisenhower under different circumstances, and Jane Fonda could likely peel off a pretty good Nancy Reagan with fuller material, but here the excellent actors playing the power figures — up to and including Alan Rickman’s Ronald Reagan — come off like a Rich Little convention. I do, however, like the scene in “The Butler” where John Cusack’s Richard Nixon pays a below-stairs visit to the White House kitchen help, trolling for support in the 1960 election. You don’t believe he’s Nixon, not for a second. Yet the scene is tense, amusingly awkward, well-written and honestly acted by, among others, Cuba Gooding Jr. and Lenny Kravitz. The encounter seems true, even if it’s cooked. All historical drama is cooked. That’s why they call it drama.

It’s up to the steady and astute performance by Forest Whitaker to keep “The Butler” from caving in under its own “Forrest Gump” sponge-of-history tendencies. This being a Daniels picture, shot every which way and going for the throat every second, grandiosity is inevitable discussing anything made by the man behind “Precious” (extremely effective) and “The Paperboy” (hilarious in its excess). So let’s put it this way: Like America itself, the movie’s a stimulating tangle.

The director, along with screenwriter Danny Strong, who wrote the Sarah Palin-HBO biopic “Game Change,” gives us a story that is a little bit true but mostly true-ish or true-esque, about a White House staffer who served several presidents before, during and after the Civil Rights movement. Whitaker portrays the fictional creation, Cecil Gaines, based very loosely on Eugene Allen, the subject of a 2008 Washington Post feature. In the introductory 1926 scenes “The Butler” illustrates what drives a quiet, watchful character deeper into himself. On this particular Macon, Ga., plantation, Vanessa Redgrave plays the matriarch, eager to train another “house slave” (although she puts it far more harshly) but just enough of a human being to be appalled by her offspring’s rape of Cecil’s mother, and the point-blank murder of his father.

Cast out on his own, Cecil soon finds himself up north and schooling himself in the ways of the hospitality industries. He is blessed and cursed with the ability to seem “invisible” while in the service of white folks. Working at a swank D.C. hotel bar, he gets his White House shot, and he’s ready. Meantime, in the scenes away from the White House, Oprah Winfrey hoards all the attention-getting material (drunken, rageful monologues, guilty philandering) as Cecil’s wife, who raises two boys while her husband spends too much time at work.

Cecil has that Gumpian knack for just being there and, with a few utterances, re-routing the river of history. A word or two in Eisenhower’s ear about segregation — bam, two steps forward. A sentence or three spoken in the presence of JFK (James Marsden), and boom — a great man acquires the courage to be even greater. With LBJ, here depicted by Liev Schreiber, an equivocating heart and mind is forever changed. (This script really is a bit silly, for all its real-world anguish.) Meantime, Cecil’s oldest, the firebrand Louis (David Oyelowo), becomes a disciple of Dr. King and then Malcolm X and, no less than his father, a witness to massive historical events.

It takes a while, but Cecil himself finally becomes a stealth agitator, nudging his employers in the direction of better pay and an occasional shot at advancement. Whitaker is such a forceful presence, you wonder initially if he’ll convince in such a recessive role. (In a Daniels film the women get all the juicy scenes and, usually, the exit zingers.) But he’s first-rate in the part, such as it is. Cecil’s conceived as a blinkered, virtually asexual man, preoccupied with appearances, unable to process who Louis has become.

“We’re trying to change the nation’s consciousness about the American Negro!” Louis lectures his father at one point, speaking like no actual revolutionary on Earth. It’s too bad “The Butler” doesn’t afford Whitaker the spacious acting opportunity that, say, “The Autobiography of Miss Jane Pittman” handed Cicely Tyson 39 years ago. On the other hand, his on-screen cohorts are no doubt happy “The Butler” tells a lot of different stories, some more effectively than others.